1. Field of the Invention
The present invention relates to an optical reproduction apparatus and an optical recording and reproduction apparatus, such as an optical disk apparatus for performing recording and reproduction of information to an optical disk in which a super-resolution film is deposited on a recording layer, and particularly to an optical reproduction apparatus and an optical recording and reproduction apparatus which can improve a CNR and can perform high speed reproduction.
2. Description of the Related Art
In an optical disk apparatus, an optical disk has been improved to achieve high density and large capacity from a compact disk (CD) to a digital video disk (DVD), and still more, large capacity has been demanded according to the improvement of the performance of computers and display devices. Particularly, an optical disk apparatus of a so-called Super-RENS (Super-Resolution Near-field Structure) method in which recording and reproduction is performed by using near-field light leaked out from a super-resolution film provided near a recording medium of an optical disk has attracted attention as a technologies suitable for achieving high density of a future optical disk since a conventional optical system used for a DVD or the like can be used and an optical disk can be made rewritable.
A conventional optical disk apparatus adopting this Super-RENS method is disclosed in, for example, “Applied Physics Letter, Vol. 73, No. 15, 1998, pp. 2078-2080” (hereinafter referred to as a document I) and “Tech. Dig. ISOM/ODS '99, TuD-29, 1999, pp. 423-425” (hereinafter referred to as a document II).
FIGS. 9A and 9B show an optical disk apparatus disclosed in the document I. This optical disk apparatus includes An aperture type Super-RENS optical disk 20A, an irradiation optical system for irradiating the optical disk 20A with a condensed light 2d obtained by condensing a parallel laser beam 2b through an objective lens 8, and a reproduction optical system (not shown) for detecting a reflected light 19a from the optical disk 20A and performing reproduction. In the aperture type Super-RENS optical disk 20A, a substrate protecting layer 22 made of SiN, a Sb super-resolution film 23a, a spacer made of dielectric layer 24 made of SiN and having a thickness of 20 nm, a phase change type recording layer 25 made of GeSbTe, and a surface protecting layer 26 made of SiN are sequentially formed on a disk substrate 21. When the Sb super-resolution film 23a is heated up to a certain temperature or higher, a phase change between crystal and amorphous occurs and absorptance of light is decreased. At the time of recording, as shown in FIG. 9A, the parallel laser beam 2b is condensed by the objective lens 8 and the Sb super-resolution film 23a is irradiated with the condensed light 2d of suitable intensity. The phase change between crystal and amorphous occurs and the light absorptance is decreased by heating the center portion of a light spot 17 of the Sb super-resolution film 23a, then a minute aperture 18a is formed at the center portion. By near-field light 19 transmitted through this minute aperture 18a, recording is performed in the recording layer 25. When the wavelength of the laser light 19 is 635 nm, and the numerical aperture of the objective lens 8 is 0.6, the diameter of the light spot 17 becomes about 0.5 μm. However, the Sb super-resolution film 23a functions as a so-called super-resolution film which enables minute recording below the diffraction limit by the objective lens 8, so that a recording mark of 0.1 μm or less which is far smaller than the diameter of the light spot 17 can be formed in the recording layer 25.
At the time of reproduction, as shown in FIG. 9B, scattering of the near-field light occurs by the minute aperture 18a and the recording layer 25, the scattered light (reflected light) 19a passes through the minute aperture 18a and almost uniformly spreads at the minute aperture 18a as a secondary point light source. The light 19b is guided with the reproduction optical system (not shown) and is incident on a photo detector (not shown), and a signal is reproduced.
FIGS. 10A and 10B show an optical disk apparatus disclosed in the document II. This optical disk apparatus includes a scattering type Super-RENS optical disk 20B, and similarly to the document I, an irradiation optical system for irradiating the optical disk 20B with a condensed light 2d obtained by condensing a parallel laser beam 2b through an objective lens 8, and a reproduction optical system (not shown) for detecting a reflected light from the optical disk 20B and performing reproduction. The scattering type Super-RENS optical disk 20B uses an AgOx super-resolution film 23b instead of the Sb super-resolution film 23a, and a substrate protecting layer 22, a spacer made of dielectric layer 24 and a surface protecting layer 26 use SiO2 as the material instead of SiN. When the AgOx super-resolution film 23b is heated up to a certain temperature or higher, Ag is extricated by a reducing reaction, so the super-resolution effect can occur as well. That is, by the above structure, similarly to the aperture type Super-RENS optical disk, when the condensed light 2d is condensed to the AgOx super-resolution film 23b, the center portion of the light spot 17 of the AgOx super-resolution film 23b is reduced so that a minute metal body 18b made of Ag is formed. This minute metal 18b scatters the condensed light 2d or a surface plasmon excited in the minute metal body 18b generates a near-field light 19c, so that recording is performed in the recording layer 25. Similarly to the Sb super-resolution film 23a, the AgOx super-resolution film 23b functions as the so-called super-resolution film which enables minute recording below the diffraction limit of condensing by the objective lens 8, so that it is possible to form a recording mark of 0.1 μm or less far smaller than the diameter of the light spot 17 in the recording layer 25.
At the time of reproduction, scattering of the near-field light occurs by the minute metal body 18b and the recording layer 25, as shown in FIG. 10B, the scattered light 19d uniformly spreads and passes through the vicinity of the minute metal body 18b. A parallel light 19b is guided by the reproduction optical system (not shown) and is incident on a photo detector (not shown), so that a signal is reproduced.
However, according to the conventional optical disk apparatus shown in FIGS. 9A and 9B, the diameter of the light spot 17 condensed to the Sb super-resolution film 23a is about 0.6 μm, while the size of the minute aperture 18a becomes 0.1 μm or less, which is as small as an area ratio of several tenths. Most of the laser light is reflected by the Sb super-resolution film 23a. The reflected light which picks up noise due to roughness or the like on the disk substrate 21 is incident on the reproduction optical system and is mixed in the reproduction signal to increase the noise, so that a CNR (Carrier to Noise Ratio) of the reproduction signal is low.
According to the conventional optical disk apparatus shown in FIGS. 10A and 10B, similarly to the above prior art, since the size of the minute metal body 18b is as small as an area ratio of several tenths as compared with the diameter of the light spot 17 condensed to the AgOx super-resolution film 23b, a laser light of a peripheral portion having intensity relatively lower than the center portion of the light spot 17 reaches the recording layer 25 and is reflected by the recording layer 25. Since the reflected light including the surface noise of the disk is incident on the reproduction optical system, the noise increases, so that there is a problem that the CNR of the reproduced signal is low.
FIG. 11 shows the CNR of the device shown in FIGS. 10A and 10B. According to the device of FIGS. 10A and 10B, as shown in FIG. 11, in recording of a minute mark of 0.1 μm or less which can not be attained by recording of a conventional system such as a CD or DVD, the CNR of a reproduced signal is 10 dB or less, which is remarkably low as compared with a CNR level of 45 dB required for signal reproduction of an optical disk, thereby signal reproduction without an error can not be made. The CNR of the aperture type Super-RENS optical disk is also at the same level or lower.
Besides, since the condensed light 2d is condensed also in the normal direction to the AgOx super-resolution film 23b and the excitation condition of resonance plasmon expected in the scattering type Super-RENS is not satisfied, a strengthening effect of scattering by the minute metal body 18b is not obtained. Since there is a problem that light intensity for recording and reproduction can not be sufficiently obtained, high speed recording and reproduction are prevented.